When a gas turbine engine is operating under transient conditions, for example when decelerating, it may be necessary to bleed air at high pressure from the core gas flow through the engine. Such air may be transferred to a bypass flow within the engine. Bleed valves are provided to control this transfer of air. The flow of bleed air from the core gas flow into the bypass flow takes place over a substantial pressure drop, and can generate significant noise. It is therefore usual to provide a noise reduction device in, or at the exit of, the flow passage between the core gas flow and the bypass duct. A typical measure is to discharge the bleed air into the bypass duct through a perforated plate, sometimes referred to as a “pepper pot” as disclosed, for example, in US2001/0042368. The pepper pot serves to break the single body of air flowing towards the bypass duct into a large number of smaller jets which promote small-scale turbulence and hence quicker mixing with the main flow through the bypass duct.